Maybe Tigase (https://tigase.net/ https://tigase.net/; https://github.com/tigase/tigase-server/ https://github.com/tigase/tigase-server/) would work for you? I

Maybe Tigase (https://tigase.net/ https://tigase.net/; https://github.com/tigase/tigase-server/ https://github.com/tigase/tigase-server/) would work for you? It does offer almost-linear horizontal scalability. (disclosure, I work for Tigase)